A polyunsaturated fatty acid (PUFA), such as eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA), has been revealed for pharmacological effects thereof recently, and has been used as a raw material for drugs and health foods. Production of a PUFA by chemical synthesis is not easy. Therefore, almost all of PUFAs utilized industrially have been produced by extraction or purification from marine organisms-derived PUFA-rich raw materials, e.g., fish oils. However, a biological raw material is a mixture of many types of fatty acids which are different in the number of carbon atoms, the number or position of double bonds, the component ratio of stereoisomers and the like from each other, and therefore a content of a PUFA in the raw material is not always high. For these reasons, it has been demanded to selectively purify a desired PUFA from a biological raw material.
In Patent Literatures 1 to 7, a method is disclosed, in which a raw material containing a PUFA and an aqueous solution containing a silver salt are brought into contact with each other to produce a complex of the PUFA and silver, then the complex is eluted into an aqueous phase, and then the PUFA is extracted from the aqueous phase with an organic solvent. In the method disclosed in Patent Literatures 1 to 7, a PUFA-containing raw material is supplied to a large amount of an aqueous silver salt solution and the resultant solution is preferably stirred to increase the chance of contact between the aqueous solution with the PUFA-containing raw material and therefore promote production of a complex of the PUFA and silver. However, in the above-mentioned conventional method, it is required to use a large-scale facility for producing a PUFA in a large amount, and a batch of an aqueous silver salt solution that has been contacted with a raw material must be collected at every extraction of the PUFA. Therefore, the conventional method is inefficient from the industrial viewpoint. In addition, in the conventional method, there are many chances of contact of the aqueous silver salt solution with oxygen and therefore deterioration of the aqueous silver salt solution may be accelerated. Furthermore, the presence of peroxide contained in a raw material may also deteriorate the aqueous silver salt solution (Patent Literature 7). The deteriorated aqueous silver salt solution is regenerated into silver after collection, and is then re-processed into a silver salt. In this manner, the deteriorated aqueous silver salt solution can be re-used. However, the cost for reproduction of silver and the cost for processing into a silver salt are expensive, and therefore use of a large amount of an aqueous silver salt solution and deterioration of the aqueous silver salt solution may increase the cost of production of a PUFA.